Wind turbine rotor blades extend longitudinally from root to tip in a ‘spanwise’ direction. The blades have an airfoil profile in cross-section, which comprises longitudinally extending leading and trailing edges. A ‘chordwise’ direction of the blade is defined as a direction perpendicular to the spanwise direction, and which lies in a plane containing the leading and trailing edges. A ‘flapwise’ direction of the blade is the direction perpendicular to both the spanwise and chordwise directions.
Wind turbine rotor blades are generally formed from two shells, a windward shell and a leeward shell, which are joined together to form the complete blade. Each shell is moulded from composite materials in a respective female mould having a shape corresponding to the shape of the shell to be produced. The terms ‘spanwise’, ‘chordwise’ and ‘flapwise’ defined above with respect to the blade are also used hereinafter to refer to the corresponding directions with respect to the moulds. The moulds themselves are made from composite materials and are supported by respective steel frames. Interface brackets are used to connect the mould to the respective frames.
Manufacturing the blades involves laying up composite material in the respective female moulds. Once the material has been laid up in the respective moulds, the moulds are placed one on top of the other and heat and pressure is applied to the closed mould assembly to cure the composite shells. During the cure cycle, the applied heat causes the blade shells and the moulds to expand relative to the steel frames. The blade shells and the moulds have a similar coefficient of thermal expansion because they are both made from similar composite material. Consequently, the shells expand at a similar rate to the moulds. However, the shells and moulds have a significantly different thermal expansion coefficient to the steel frames, causing the shells and moulds to expand more than the steel frames.
In order to avoid distortion of a mould during a cure cycle, it is known to use interface brackets that accommodate relative movement between the mould and the supporting structure, which is caused by these differential rates of thermal expansion. Background art is discussed briefly below.
WO2011/029273A1 describes an interface device for adjusting the shape of a rotor blade mould in the chordwise direction. The device is configured to accommodate thermal expansion of the mould relative to a frame in the spanwise direction. The chordwise shape adjustment prevents the device from accommodating thermal expansion in the chordwise direction.
U.S. Pat. No. 4,398,693 describes a device for fastening a rotor blade mould to a supporting structure. The device is configured to accommodate thermal expansion of the mould relative to the supporting structure in both spanwise and chordwise directions.
WO2006070013A1 describes interface brackets for coupling a mould to a supporting structure. A plurality of brackets are arranged radially with respect to an expansion centre to accommodate spanwise and chordwise expansion of the mould relative to the supporting structure.
It is important that the mould is precisely aligned with the supporting structure in order for the interface brackets successfully to accommodate relative movement between the mould and the supporting structure. In practice, hundreds of interface brackets may be required for a wind turbine blade mould, as the blades of modern utility-scale wind turbines are very large, typically in excess of sixty meters in length. Achieving such precise alignment at each bracket is challenging and time consuming.
Against this background, the present invention aims to provide an improved interface bracket.